Photovoltaic devices are known in the art (e.g., see U.S. Pat. Nos. 6,784,361, 6,288,325, 6,613,603 and 6,123,824, the disclosures of which are hereby incorporated herein by reference). Amorphous silicon (a-Si) and CdTe type photovoltaic devices, for example, each include a front contact or electrode. Typically, the front electrode is made of a transparent conductive oxide (TCO) such as tin oxide or zinc oxide formed on a substrate such as a glass substrate. Accordingly, it will be appreciated that zinc oxide doped with Al (ZnAlOx) is a known TCO material for use as an electrode for a photovoltaic device. In certain applications, such as CdTe photovoltaic devices as an example, high processing temperatures (e.g., 550-600 degrees C.) are used during manufacturing. Zinc oxide is also used in applications such as low-E coatings to support IR reflecting layers which may be made of silver or the like.
Unfortunately, conductive ZnAlOx tends to lose a significant amount of its electrical conductivity when heated above about 400 degrees C. This loss of conductivity may be caused by fast oxygen migration from grain boundaries into the bulk of the crystallites. At even higher temperatures (e.g., 625-650 degrees C.), structural transformation of zinc oxide starts to occur, which is particularly disadvantageous for applications such as heat treatable low-E coatings because it compromises the integrity and corrosion resistance of IR reflecting films that are often formed directly on and over the zinc oxide; such a transformation may be detected for instance by an increased <002>/<103> XRD peak ratio.
It is apparent from the above that there exists a need in the art for an improved TCO material. In certain example embodiments of this invention, there exists a need in the art for a zinc oxide, or zinc aluminum oxide, based TCO that has a reduced potential for significant conductivity loss at high temperatures (e.g., above about 400 degrees C., or possibly even higher). In certain example embodiments of this invention, there exists a need in the art for a zinc oxide, or zinc aluminum oxide, based TCO that has a reduced likelihood of structural transformation at high temperatures. Such improved TCO materials may be used in various applications, including but not limited to electrodes (e.g., front electrodes) in photovoltaic devices, as silver-supporting layers in low-E coatings, and the like.
It has been found that by doping a zinc oxide, or zinc aluminum oxide, based TCO with small amounts of yttrium (Y), the TCO can be improved in one or more respects. For example, by doping a zinc oxide, or zinc aluminum oxide, based TCO with a small amount of yttrium (Y), the resulting TCO can realize a reduced conductivity loss at high temperatures. As another example, by doping a zinc oxide, or zinc aluminum oxide, based film (which is a TCO in certain preferred instances, but need not be in all cases) with a small amount of yttrium (Y), the resulting film can realize reduced or no structural transformation at high temperatures (e.g., of at least about 400 degrees C., or even possibly of at least about 550, 600 or 625 degrees C.).
In certain example embodiments, a transparent conductive oxide (TCO) based front electrode for use in a photovoltaic device is of or includes zinc oxide, or zinc aluminum oxide, doped with yttrium (Y). In certain example embodiments, the addition of the yttrium (Y) to the conductive zinc oxide or zinc aluminum oxide is advantageous in that potential conductivity loss of the electrode can be reduced or prevented. As used herein, the term “yttrium” includes and covers both metallic yttrium, as well as yttrium oxide such as Y2O3 or any other suitable stoichiometry.
Moreover, in certain example embodiments of this invention, the electrode (e.g., ZnOx:Y or ZnAlOx:Y) may be sputter-deposited in a non-stoichiometric oxygen deficient form, or may be deposited in any other suitable manner. Sputtering at approximately room temperature may be used for the deposition of the electrode in certain example instances, although other techniques may instead be used in certain instances. For example, the electrode may be sputter-deposited using a ceramic target(s) made of ZnOx or ZnAlOx doped with Y and/or Y2O3. Alternatively, the electrode may be sputter-deposited in an oxygen gas (and possibly argon gas, or any other suitable gas) inclusive atmosphere using a metal or substantially metal target(s) made of Zn or ZnAl doped with Y; the gas composition or mixture may be chosen so as to make the initially deposited material substoichiometric in certain example instances.
In certain example embodiments, the electrode of or including ZnOx:Y and/or ZnAlOx:Y may be used as any suitable electrode in any suitable electronic device, such as a photovoltaic device, a flat-panel display device, and/or an electro-optical device.
In certain example embodiments of this invention, the TCO (e.g., ZnOx:Y or ZnAlOx:Y) electrode or film may have a sheet resistance (Rs) of from about 7-50 ohms/square, more preferably from about 10-25 ohms/square, and most preferably from about 10-15 ohms/square using a reference example non-limiting thickness of from about 1,000 to 2,000 angstroms, although other thicknesses are possible, especially smaller thicknesses may be used in low-E applications.
Sputter deposition of a TCO (transparent conductive oxide) at approximately room temperature for a front electrode in a photovoltaic device would be desirable, given that most float glass manufacturing platforms are not equipped with in-situ heating systems. Moreover, an additional potential advantage of sputter-deposited TCO films is that they may include the integration of anti-reflection coatings, resistivity reduction, and so forth. For example, a single or multi-layer anti-reflection coating may be provided between the glass substrate and the TCO front electrode in photovoltaic applications. As another example, a silver based IR reflecting layer may be sputter-deposited over the ZnOx:Y or ZnAlOx:Y in low-E coating applications.
In certain example embodiments of this invention, there is provided a photovoltaic device comprising: a front glass substrate; a semiconductor film; an electrically conductive and substantially transparent front electrode located between at least the front glass substrate and the semiconductor film; and wherein the front electrode comprises zinc oxide and/or zinc aluminum oxide, doped with from about 0.001 to 5.0% yttrium.
In other example embodiments of this invention, there is provided an electrode structure for use in an electronic device, the electrode structure comprising: a substrate; an electrically conductive and substantially transparent electrode supported by at least the substrate; and wherein the electrode comprises zinc oxide and/or zinc aluminum oxide, doped with from about 0.001 to 5.0% yttrium
In still further example embodiments of this invention, there is provided a coated article comprising: a low-E coating supported by a glass substrate; the low-E coating comprises at least one IR reflecting layer, with the IR reflecting layer being provided over a layer comprising zinc oxide and/or zinc aluminum oxide, the layer comprising zinc oxide and/or zinc aluminum oxide being doped with yttrium.